The present invention relates to methods and apparatus for the preparation of microalgae and microalgal products, and, more particularly, in a system for drying microalgae/microalgal products which have been harvested from an algal culture media-processing source and strained of excess water by a spray drying process it relates to an improvement by which oxidation of the microalgae/microalgal products is minimized and carbon dioxide is recovered and recycled to the algal culture media-processing source comprising, a burner for producing heated drying gas in which metered input air for combustion is mixed with fuel at or before a dryer gas heating burner and which does not require oxygen in drying gas which is heated; spray dryer means for using the heated drying gas from the burner to dry the microalgae/microalgal products; cyclone means for separating dryed microalgae/microalgal products from moist drying gas; means for scrubbing and dehumidifying the moist drying gas; means for recycling scrubbed and dehumidified drying gas to the dryer gas heating burner; means for venting a portion of the drying gas equivalent in volume to combustion gases created in the burner; a tank having an inlet receiving a flow of algal culture media from the algal culture media-processing source and an outlet returning the flow of algal culture media to the algal culture media-processing source; means for introducing the vented portion of the drying gas into the tank; and, means for mixing the portion of the drying gas through the algal culture media in the tank.
Microalgae are a potential source of food, fuel, chemicals, and pharmaceuticals. Microalgae are typically harvested from culture systems by screening, filtration, sedimentation, and centrifugation, or a combination thereof. Once harvested, microalgal cells contain from 85% to 90% water. The high water content is due to internal water in the cells that cannot be removed by mechanical means. As a result, drying of microalgae according to known prior art techniques is energy intensive and costly, accounting for as much as 30% of the production cost. In addition, many components of microalgae, such as beta-carotene and enzymes, are quickly oxidized if exposed to oxygen during the drying process.
Numerous methods have been used to dry microalgae and their products. Richmond et al. review drying methods applicable to microalgae in the Handbook of Microalgal Mass Culture as do Borowitzka et al. in Micro-Algal Biotechnology. These methods include sun drying on sand, sun drying on plastic sheets, drum drying in single and double drum dryers, and spray drying. Spray drying has the advantage of producing finely powdered microalgal products which are easily tabletted or blended with other components.
Masters reviews the present state-of-the-art of spray drying and also recommends methods applicable for microalgae in the fifth edition of the Spray Drying Handbook. For drying microalgae, Masters recommends an open-cycle spray dryer with a particular direct-fired air heater. As with drum drying, open-cycle spray drying with direct-fired air heaters exposes microalgae to elevated temperatures (140.degree.-200.degree. F.) and high oxygen concentrations (19-20%); and, offers no means for recovery of carbon dioxide produced in the dryer's burner.
In the Spray Drying Handbook, process layouts are discussed for spray drying. In particular, a semi-closed-cycle layout with a direct air heater is presented. The advantage of this type of spray dryer layout is that, with the proper type of burner, oxygen can be virtually eliminated from the recycled drying gas. Hence, oxygen-sensitive products are not oxidized during the drying process. The semi-closed-cycle layout and all other prior art closed-cycle layouts use a scrubber to remove product dust from the recycled drying gas and a separate condenser to dehumidify the recycled drying gas. There is also no mention or suggestion of recovery of carbon dioxide produced by combustion in the dryer burner.
Wherefore, it is the object of this invention to provide a process and associated apparatus for drying microalgae and microalgal products by employing a semi-closed-cycle layout wherein a low oxygen content is maintained in the recycled drying gas and wherein carbon dioxide is recovered from the recycled drying gas.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.